The present invention relates to the correction of truncation of transmission computerized tomography (TCT) data.
A single photon emission CT (SPECT) apparatus, which is one type of nuclear medical diagnostic apparatus, detects gamma (xcex3) rays emitted from radioisotope (RI) injected into the human body under examination and measures the RI distribution within the human body. The gamma rays decay within the human body. Thus, qualitative measurement requires measuring how gamma rays decay in areas of the human body through transmission CT using an external gamma ray source and correcting the RI measurements accordingly.
Of TCTs using an external gamma ray source, the TCT using a fan-beam collimator, i.e., the TCT that uses a fan beam not a parallel beam, collimates gamma rays at two places: one near the source and one near the detector. Collimating the gamma rays reduces the effects of scattered rays, obtaining TCT data useful in measuring the distribution of coefficients of decay of gamma rays within the human body.
However, the fan beam-based TCT, while being little affected by scattered rays, has a disadvantage of the effective field of view being small in comparison with the parallel beam-based TCT. In measuring the body of an object under examination, therefore, a part of the body may protrude from the effective field of view.
The reconstruction of projection data without taking into consideration that a part of the body under examination is protruding from the effective field of view would produce artifacts due to imperfect reconstruction resulting from truncation errors.
Thus, high-precision TCT data cannot be obtained, which leads to a failure to take a measurement of the distribution of coefficients of decay of gamma rays with precision in SPECT using TCT data.
A conventional technique related to truncation correction of TCT data is described in xe2x80x9cReduction of Truncation Artifacts in Fan Beam Transmission by Using Parallel Beam Emission Dataxe2x80x9d by Tin-Su Pan, Michael A. King, et al., IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 42, NO. 4, AUGUST 1995. This conventional technique extracts the contours of the body and the lung from SPECT data to obtain outline information and uses the outline information in truncation correction of TCT data. However, this technique is not practical. The reason is that the extraction of the contours of the lung from SPECT data is difficult and tentative coefficients of decay of gamma rays have to be used.
It is therefore an object of the present invention to provide an apparatus and method which allow precise TCT data having truncation errors corrected to be obtained.
According to the present invention, there is provided a truncation correction apparatus for transmission CT comprising: first acquiring means for acquiring emission data; second acquiring means for acquiring TCT projection data; calculating means for calculating a center of gravity of a TCT image and a total of the TCT projection data from the emission data and the TCT projection data; and correcting means for correcting the TCT projection data on the basis of the center of gravity of the TCT image and the total of the TCT projection data.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.